Various devices for mixing synthetic plastics, resins or polymers and their additives, which are added during the mixing process or premixed therewith, and which are especially present as fillers, lubricants, stabilizers, dyes, pigments, antistatic agents and the like, are well-known in the industry. The mixing elements used in such devices differ in construction depending upon whether a dispersive mixing (shearing, dividing) or a distributive mixing (distributing, blending) is sought to be achieved. See generally Schiedrum, Kunststoffe, vol. 63, No. 6, pp. 355-61 (1973). However, the object of such mixing is always the uniform distribution of all elements and additives, possibly added in only very small concentrations, in the base substance. It is possible for the finished mixture to take the form of a material with a solid consistency, for example as in an agglomerate, or as a homogeneous melt.
Such known mixing apparatus as are shown, for example, in German patents DT-PS 902,789, DT-PS 1,197,438, DT-PS 1,198,051 and German published application DT-AS 1,529,964, have radially intermeshing mixing or kneading elements, with one set of elements being arranged about a rotor of a screw extruder and rotating therewith, and another set of elements being mounted in a fixed position on the casing enclosing the rotor.
Mixing apparatus of the above type have an inherent drawback in that the design and arrangement of the mixing elements or disk packs do not allow for the simple disassemblage of the rotor for cleaning, inspection, repair or the like without a relatively great expenditure of time and effort spent in the dismounting and reassembly. In particular, if the radially intermeshing mixing elements are alternately mounted on the rotor and casing, rapid ejection of the rotor is not possible in any event, thereby rendering the apparatus unsuitable for the mixing of rigid polyvinyl chloride since such rapid ejection is necessarily required for the protection of the machine in the event of a power failure when working such material.
An additional disadvantage of mixing apparatus of the type disclosed in DT-AS 1,529,964 is that in order to improve the mixing effect by arranging several mixing elements on the rotor and in the casing, defined gaps must be present between the revolving and fixing mixing elements so as to assure proper mechanical functioning of the device and avoid jamming or butting of axially opposed annular surfaces. However, these axial gaps undesirably bring about variable loading of the material to be mixed in both mechanical and thermal respects and, in consequence, an uncontrolled and unpredictable flow process which prevents the optimal reproducible mixing of the materials being treated. For example, the varying mixing times of such devices in the case of thermally sensitive plastics or resins will lead to damage of mixed materials and to an inhomogenous and generally non-uniform quality of the resulting product.
In addition to the above-described devices, there are also known mixing apparatus of the type disclosed in German patent DT-PS 1,037,698 in which systems of grooves or furrows lie opposite one another and extend in axial directions along a drivable cylindrical rotor having little radial play and a surrounding casing. These groove systems are separated circumferentially by shear crosspieces or lands which do not engage one another.
However, in spite of the advantage of rapid disassembly of such devices through the axial ejection of the rotor from within the casing, such apparatus exhibit certain drawbacks in the mixing process in that the shearing and mixing of the materials with their additives takes place in an inherently uncontrollable manner and only relatively large agglomerates are ground or pulverized between the shear lands. The material passing through such devices will be essentially sheared only once in the longitudinal direction and distributed on the grooves of the rotor and casing. Beyond this initial shearing, it is left to chance whether the particles will be again subjected to a discrete shearing action as they proceed through the device since it is not necessary that radial flow of the particles take place beyond this point between the grooves and shearing lands of the rotor and casing.
A further disadvantage experienced with such mixing devices is that the groove systems are not self-cleaning so that stoppages often times occur, to the detriment of the homogeneity of the mixture and resulting product quality, as a result of, for example, a build-up of insufficiently melted agglomerates in such grooves.
The above-discussed disadvantages can largely be eliminated through an arrangement of the groove systems in the manner disclosed by U.S. Pat. No. 3,174,185. With such an arrangement several groove systems are mounted on the rotor and in the casing in axial succession and are axially shifted with respect to one another. Due to this axial interruption of the grooves, there is created in a simple manner a forced flow of the material back and forth between the groove system of the rotor and the groove system of the casing which insures improved shearing and mixing of the material as it flows through the apparatus. However, the production of such groove systems in the rotor and casing is quite expensive due to the extensive and difficult machinery required. This is especially a problem when the inside diameter of the casing is relatively small.